Well, booting on UEFI systems requires the drive be formatted with UEFI support which obviously your’s wasn’t. Reading a USB drive on a UEFI system simply requires it be in a format Windows recognizes; ie FAT, exFAT or NTFS. Obviously your flash drive doesn’t have one of those formats, and since you say it has a Linux system on it it is probably formatted in a Linux ext format.
So, the question is, what do you want to do with the drive. If you want the files on it you need to find a non-UEFI machine, boot the Linux system and copy all the files to the machine’s hard drive, reformat the flash drive to a format Windows supports, then copy the files from the hard drive back to the flash drive.
If you want to boot the drive on a UEFI machine, to a Linux system, the easiest approach, after you backup your files, is to download a utility named Easy2Boot and use to make the flash drive bootable and put a Linux system on it. Another option, after you backup your files, is to put Porteus on the flash drive, it supports booting on UEFI systems.
